The present invention relates to a method and an apparatus for correcting digital code words in a digital communications network, in particular in a communications network in which what is referred to as RBS signaling is carried out.
As is shown in FIG. 6, communications networks 2, in particular such as telephone networks, are frequently used as interfaces for transmitting communication information between a digital modem 1 and an analog modem 5, or what is referred to as a linecard. The digital communications network 2 may in general, for example, be in the form of an integrated services digital network (ISDN), an optical waveguide network, a coaxial cable network, a satellite network, or else may be configured without the use of wires. The communication via the digital communications network 2 generally makes use of pulse code modulation (PCM). The digital communications network 2 is connected via a coder/decoder 4 (Codec) to the analog modem 5, with the Codec 4 being used as the interface between the digital communications network 2 and the analog modem 5. In order to transmit communication information from the digital communications network 2 to the analog modem 5, the Codec 4 has, inter alia, a digital/analog converter for converting nonlinear signal levels, for example xcexc-coded or A-coded signal levels, to a linear analog signal, and, for transmitting communication information from the analog modem 5 to the digital communications network 2, it has, inter alia, an analog/digital converter for converting a linear analog signal to non-linear xcexc-coded or A-coded code words.
An in-band signaling technique referred to as RBS (Rob Bit Signaling) is frequently used in digital networks. The RBS technique is a signaling method that is used especially in North American telephone networks, with the least significant bit (LSB) of a digital PCM code word that is to be transmitted via the telephone network being changed at specific intervals, and being used for signaling purposes. For example, the LSB in every 6th to 24th frame is thus set either to the logic value xe2x80x9c0xe2x80x9d, or to the logic value xe2x80x9c1xe2x80x9d.
For transmissions based on the latest V.90 Standard modem, the transmitted PCM values must be reproduced as accurately as possible. However, RBS channels corrupt the PCM values, thus making correct identification in the modem harder. It is thus necessary to set the bits which are changed when using the RBS technique to a constant value before such data is received by the Codec 4 shown in FIG. 6. However, in practice, this is frequently not the case.
Furthermore, the use of the RBS technique is associated with further problems. Particularly when using the V.90 standard for transmitting PCM modem data, adaptation to transmission paths which continue onward in the LSB signaling information is difficult. The RBS technique increases the error rate. The maximum possible error when using the digital modem 1 to code a given signal level is 0.5*LSB. The LSB is forcibly set either to 0 or 1 in an RBS frame or an RBS channel. The maximum error caused by the coding process is thus increased to three times this value, that is to say to 1.5*LSB. The performance of the modem is thus necessarily worse when using the RBS technique.
There is thus a need to improve the data transmission in digital communications networks in which the RBS technique is used.
For this reason, Codec modules 4 are known to which the information about when an RBS channel or an RBS connection is present is supplied via an external trigger signal, in response to which the PCM characteristic for decoding the received digital code words is appropriately changed and adapted. Thus, for example, the error produced by the RBS technique can be reduced by ignoring the LSB for RBS frames and, instead of this, by setting the analog output value to half the two adjacent PCM values. However, this procedure has the disadvantage that, firstly, an additional control signal must be provided by means of appropriate hardware and, secondly, only local RBS channels can be coped with. However, it is possible for a situation to occur in which a number of line sections in a transmission path use the RBS technique, with these line sections not being synchronized and influencing various digital PCM values to be transmitted.
RBS compensation systems for compensating for the disturbances associated with the use of the RBS technique have been proposed in U.S. Pat. No. 5,754,594, U.S. Pat. No. 5,784,405 and U.S. Pat. No. 5,812,075, in which case, according to these documents, the corresponding RBS compensation system is in each case told which frames or PCM values in the digital data to be transmitted have RBS applied to them by means of the digital communications network via which they are transmitted. These solutions thus presupposed that the position and the occurrence of the RBS channels or RBS values are known although, frequently, this is not true. Furthermore, the systems described in these documents are used exclusively to minimize the faults in an analog signal caused by RBS channels. However, they are not able to satisfy the requirements, which have already been mentioned above, associated with the latest V.90 Standard/modem.
The present invention is thus based on the object of providing a capability for reproducing received code words or PCM values as accurately and reliably as possible even when they have been corrupted by the RBS technique, with this being intended to be possible, in particular, even when the position of the RBS channels is not known.
According to the present invention, this object is achieved by a method having the features of claim 1, and by an apparatus having the features of claim 12. The dependent claims define advantageous and preferred exemplary embodiments of the present invention.
According to the invention, training code words transmitted during a training phase are evaluated, and the presence of an RBS channel is deduced as a function of the result of the evaluation of the training code words. The position of each RBS channel and/or the nature of the RBS channel, that is to say the nature of the RBS technique, can then be stored, so that the knowledge obtained during the training phase is also available during normal operation for correction of the RBS channels or of the corresponding code words.
When an RBS channel is identified, the code words transmitted via this channel are corrected such that that bit in this code word which is changed by the RBS technique is always set to a fixed value and is output (to a downstream Codec), so that the initially mentioned requirements can be satisfied after setting the changed bits to a constant value, before they are supplied to the Codec. The present invention thus makes it possible to achieve an improved transmission rate using the analog modem.
According to a first exemplary embodiment, the code words in identified RBS channels are corrected such that they are preferably supplied to the Codec with the fixed value xe2x80x9c1xe2x80x9d.
A second exemplary embodiment makes use of the fact that the training code words are transmitted repeatedly during the training phase, so that RBS channels can be detected by comparing the training code words transmitted during the various repetitions and corresponding training code words.
The code words associated with RBS channels can also be replaced by their original values, and can be supplied to the Codec such that the influences of the RBS technique are eliminated. This thus reliably prevents corruption of the training signal by the RBS technique which, for example in the case of the V.90 Standard, could lead to an incorrect decision on the V.90 capability of the receiving modem.